In order to implement both of increasing an operation frequency and downsizing/reducing power consumption of a PLL circuit, there are cases where an ILFD is used as a prescaler to divide a frequency of output from a voltage controlled oscillator (VCO). The ILFD is based on a phenomenon called injection locking. When an oscillator having a free-run frequency is input with a signal externally, the oscillator outputs a signal in synchronization with a frequency equivalent to that of the input signal divided by an integer. As compared to frequency dividers of an emitter coupled logic (ECL) type or current mode logic (CML) type which have been conventionally in use as a prescaler, the ILFD has an advantage that an operation frequency can be increased and that current consumption can be reduced.
Meanwhile, the ILFD has a characteristic that the ILFD can operate only around the free-run frequency. Therefore, there is a problem that, for example when characteristics of an element varies due to production conditions, temperature, or a source voltage and thereby the free-run frequency of the frequency divider deviates from a desired operation frequency, operations of frequency division may be disabled.
In order to solve this problem, a method has been proposed for measuring a free-run frequency of an ILFD including an adjustment circuit of the ILFD and having an adjustment means of the free-run frequency, calculating a difference from a desired frequency, and thereby controlling the free-run frequency of the ILFD (Patent Literature 1).